This invention relates to a static pressure automatic control device capable of preventing excessive increase of static pressure inside a closed-circuit type respirator and securing the efficacy thereof.
For rescue and fire extinguishing operations in fire, the respirators of compressed air (oxygen) open-circuit type have been widely used. This type of respirators however is uneconomical as the user inhales air (oxygen) contained in a pressurized container but the exhaled air is discharged to outer atmosphere and wasted. They are detrimental in that the serviceable time is quite limited, too. The respirators of closed-circuit type aim to solve these prior-art problems by passing exhaled air into a cleaning agent instead of discharging it to outside, so as to remove carbon dioxide therein by absorption, and automatically supply oxygen to the remaining air in an amount compensating for the amount consumed from the original oxygen amount, and circulate the air for respiration again. This device can withstand a long service time as compared with the compressed air (oxygen) open-circuit type.
FIG. 1 shows a type of the prior art closed-circuit respirator. In the respirator, an inhalation pipe 5 and an exhalation pipe 3 are connected to a mask 1 inhalation valve 2 and exhalation valve 4. The exhaled air which is discharged through the exhalation pipe 3 is passed to a cleaning canister (carbon dioxide removing device) 7 which is filled with an absorbent agent (such as Ca(OH.sub.2).sub.6 to remove carbon dioxide gas, and the cleaned air is passed through an inhalation box 9 housing a bag 8 which communicates to open air, and then returned to the inhlation pipe 5. The bag 8 is made of a thin elastic material and inflates during inhalation and deflates during exhalation. Oxygen is supplied from a compressed oxygen cylinder 10 to the inhalation box 9 via a demand valve 11 in an amount corresponding to the removed carbon dioxide. The respirator is so structured that if the pressure inside the mask 1 increases excessively, air should be let out through an automatic relief valve 12.
The internal pressure in the circulation system of a respirator fluctuates responding to each breathing; i.e. it generally becomes negative pressure at inhalation and positive pressure at exhalation. The range of pressure that a respirator should cover may vary widely depending on breathing conditions but may generally be more than +/-30 mm in water column. The pressure variable by respiration is defined herein as dynamic pressure while the internal pressure which is not directly related to respiration is defined as static pressure. The static pressure within the circulation system of a respirator may increase because of the increase in internal pressure which is induced by the heat generated by chemical reaction in the absorbent agent, excessive discharge of oxygen into the circulating system or a reduction in pressure from a pressurized state to the atmospheric pressure. This sometimes makes the bag 8 deflate beyond the normal deflatable margin as the breathing bag 8 repeatedly inflates and deflates within the inhalation box 9. In such a case, the breathing bag 8 which is generally made of a rubber coated fabric of ca. 0.2 mm thickness tends to be twisted or otherwise deformed beyond restorable extent, which leads to malfunction thereof. The excessive increase of static pressure therefore should be avoided by all means. According to the result of laboratory tests, such increase usually remains within +10 mm in water column. As the fluctuation of static pressure is smaller than that of respiratory dynamic pressure which is ca. more .+-.30 mm, if the increment of static pressure is attempted to be offset by letting out the pressure via an automatic relief valve 12, the dynamic pressure will also be discharged to outside, leading to excessive working of the pressure reducing device and therefore excessive supply of oxygen.